Wireless communication devices are typically configured to operate in an assigned frequency band within an allocated spectrum of available frequency bands. The allocated spectrum can be a contiguous frequency span or may consist of multiple disjoint frequency spans. In some systems, the frequency assignment can be dynamic and can change during the course of communications, such as in the case of a handoff in a multi-mode cellular communication system.
Wireless communication devices typically incorporate a Voltage Controlled Oscillator (VCO) that can be tuned across a tuning range that enables the device to operate in any assigned frequency band from the multiple frequency bands. The VCO is typically incorporated into a frequency synthesizer that can include a phase locked loop (PLL) that is configured to maintain a control voltage of the VCO at a value that tunes the VCO output frequency to a desired frequency.
Because the frequency response of the VCO typically varies over an operating temperature range of the VCO, the PLL typically provides different control voltage values over temperature to achieve the same desired VCO frequency. The change in a free running VCO output frequency for a given control voltage value can be referred to as a temperature drift, and may be characterized in terms of ppm/° C. A large VCO temperature drift increases the range of control voltage that is required to maintain a given VCO output frequency over temperature.
The design of wireless communication devices is complicated by the continual desire to decrease the form factor of portable devices. For example, the size of cellular telephones has decreased from a volume the size of a small briefcase to a volume that fits easily within the palm of a hand. The shrinking form factor of wireless communication devices shrinks the volume available for portable power sources, which are typically batteries. As such, power consumption is typically of concern in wireless communication devices, and devices are designed to minimize power consumption, thereby maximizing battery life.
One way in which battery power can be decreased is by designing internal components that operate at lower voltage levels. It is not uncommon for the integrated circuits within a cellular telephone to operate at 2.2 volts or less.
The design constraints of decreased size and decreased operating voltage in conjunction with decreased power consumption create further issues for a VCO in a wireless communication device. The size of a VCO can be minimized by implementing a VCO within an integrated circuit. VCO designs have utilized external components, such as the reactive components used in the resonant circuit of the VCO, that can be relatively large compared to the rest of the VCO.
However, unless a step up voltage converter is used, the control voltage for tuning the VCO output frequency is limited to less than the supply voltage. However, the use of a step up voltage converter is typically not desirable as additional space is required. Furthermore, the converter cannot operate at 100% efficiency, resulting in additional power consumption. Thus, the control voltage available to tune the VCO output frequency is typically less than the supply voltage.
The problem of VCO frequency drift adversely affects the ability of the VCO to tune across a desired spectrum, because a portion of the control voltage tuning range is set aside to compensate for frequency drift.